1. Field of Invention
This invention relates generally to a method and apparatus for the processing of a signal having cyclostationary properties and, more particularly, to a method and apparatus for estimating the transfer characteristics of a medium from the output of the medium and using the estimated transfer characteristics to improve the signal to noise ratio of a recovered cyclostationary signal.
2. Description of the Prior Art
Since World War II, there has been an increasing interest in propagation of a signal with a direct sequence spread baseband spectrum. The increasing interest persists because at any given level of radiation, spreading causes a reduced power density over the spread spectrum. Additionally, encryption is almost inherent in the spreading. Because of the reduced power density and the encryption, a multiplicity of sources may simultaneously radiate at frequencies within the spread spectrum without interfering at a receiving station. Since the multiplicity of sources may simultaneously radiate without interfering, the spread spectrum may be used to achieve what is referred to in the art as code division multiple access.
More particularly, the spread spectrum may be produced by modulating an unspread baseband signal with the output of a spread spectrum sequence generator. Typically, the sequence generator generates a spreading signal having a level representative of either one or minus one at any given time. The spreading signal is most often referred to as a sequence of chips, each chip consisting of a period during which the spreading signal is of either a positive (one) or negative (minus one) polarity. It should be understood that the sequence is cyclically generated.
The rate at which the chips are provided is known as the chip or clock rate. Additionally, the sequence has a defined pattern, known as the spread sequence. The defined pattern is the encryption referred to hereinbefore.
The modulation of the unspread baseband signal provides a spread signal having an amplitude which is the product of the level of the spreading signal and the amplitude of the baseband signal. The spread signal has an increased spectral width that substantially equals the spectral width of the spreading signal.
Since all of the chips have substantially identical pulse shapes, the spread signal has statistical characteristics that vary cyclically. Hence, the spread signal is referred to in the art as a cyclostationary signal.
It should be understood that a signal, substantially the same as the unspread baseband signal, is recovered by modulating the spread signal with a local spreading signal provided by a local spread sequence signal generator at the receiving station. The output of the local sequence generator must be synchronized to the received signal to accomplish the recovery.
The baseband signal may be an audio signal, the output of a Phase Shift Keyed (PSK) generator, a light signal, or the output of any of a plethora of generators used to provide signals for propagation through a medium. Moreover, the medium may be the atmosphere, the earth, a glass fiber or any other medium.
Transmission through the medium may cause distortion of the spread signal. The distortion manifests itself as a degradation of the signal to noise ratio of the recovered baseband signal. It is well known that the degradation is reduced by appropriately modifying the local spreading signal.
An appropriately modified local spreading signal is obtained by altering the local spreading signal in accordance with the transfer characteristics of the medium. However, the transfer characteristics may be unknown and may be subject to temporal changes. It is desirable to determine the transfer characteristics from the output of the medium, and use it to improve the signal to noise ratio of the recovered baseband signal. This invention covers those spread and unspread signals which are included or encompassed by the term cyclostationery signals.